metrics: sstables_per_read_histogram returns number of sstables reads

[Deexie]

EstimatedHistogram, which is responsible for sstablesPerRead column in SSTablesPerReadHistogram table histogram, returns values indicating the number of sstables reads instead of the number of sstables.

[mykaul]

We don't have the 'backport candidate' label here - unsure what the process is later on, since we do wish to have this fix in earlier releases as well (do we update the submodule?). @DoronArazii - need to follow up once this is merged.

[DoronArazii]

@DoronArazii - need to follow up once this is merged.

It's in master --> https://github.com/scylladb/scylladb/commit/50ee4033a92e395e70c08e64d60667bb2681fd2b

[denesb]

@DoronArazii - need to follow up once this is merged.

It's in master --> scylladb/scylladb@50ee403

It is actually scylladb/scylladb@f82287a9afbdaeb87138a0918f05f4c52e9b5966.